1. Field of the Invention
The present invention relates to a method for separation of polyvinyl alcohol contained, for instance, in the waste water of the treatment of textile products.
2. Description of the Related Art
In the textile industry, polyvinyl alcohol (hereinafter will be described in short as PVA) is used widely, for instance, as a sizing agent for preventing the thread breakage in weaving. The PVA for this purpose is dissolved in the washing bath in the course of desizing and scouring, and accordingly the COD load in the drainage is unavoidably increased. Further, since PVA can hardly be decomposed due to the action of bacteria, when it is discharged into the river or the sea as it stands without being decomposed, the amount of oxygen dissolved in water becomes unavoidably decreased, and the death of the organisms in water is caused to occur due to the break of the balance in the ecological system. Thus, the sea and river are turned to be deathlike. Accordingly, severe rules have been imposed on the discharge of the waste water containing PVA.
As a measure in compliance with the rules for the waste water containing PVA, therefore, separation of PVA by coagulation with the addition of a chelate compound of boron or a coagulant comprising a high polymer organic compound or a suitable inorganic compound has been adopted in the art. However, in such a method for the separationrecovery of PVA, the formation of sludge is remarkable, the recovery rate of PVA is low and moreover no satisfactory result can be obtained in the point of COD regulation. For instance, in the conventional method of the separation with the use of a chelate compound of boron, although the recovery rate is high as about 90%, the separated product contains a very large amount of water reaching to about 90% and the amount of thus separated PVA is very large, so that its disposal is very difficult. Moreover, when the product is left as it stands, the product returns to a pasty state due to the separation of the chelate by the change of pH, and its treatment is very troublesome. In the case of the separation with the use of an organic or an inorganic coagulant, the recovery rate is very low as about 50%, leaving a large amount of sludge, and there is also a difficulty in its disposal.